Final answer:
Pressing the GEN 1 LINE pushbutton with OFF illuminated disconnects generator 1 from the aircraft's electrical system, enabling pilots to manage power sources.
Step-by-step explanation:
When the GEN 1 LINE pushbutton is pressed and OFF is illuminated on the EMER ELEC PWR PANEL, it typically means that the connection of generator 1 to the respective electrical bus has been opened, and this generator is no longer supplying power to the aircraft's electrical system.
This action might be taken during troubleshooting procedures or emergency situations where isolating an electrical generator is required. By pressing this pushbutton, pilots can manage the aircraft's electrical system power sources and ensure the safe operation of the system.